Cannot see admin icon when I login

I recently purchased Vaio with Vista Home premium.

I changed the original administrator account to the standard account after I
changed an existed account to administrator.

After I restart the PC, whenever I try to manage the settings, pop-up
message shows up asking the password for administrator. After I type in the
password, the error message comes out, saying " Logon failure; the user has
not been granted the requested logon type at this computer ".

What can I do to fix it? I tried restarting with safe mode, too. But it
didn't ask login name, so I couldn't login with administrator. Do I need to
reinstall Windows? The machine didn't come with the operating system CD.

PLEASE HELP ME !!